With the expanded use of telephones by both business and private users, many subscribers have developed the need, for billing and/or cost control purposes, to monitor the telephone activity on each of the lines of their telephone system. For the telephone subscriber with multiple telephone lines, but not a sufficient number of telephones to warrant the use of a PABX (Private Automatic Branch Exchange), the presently available monitoring systems usually require separate units for each telephone. Such systems are expensive and impractical since they require duplication of equipment for each telephone.
U.S. Pat. No. 3,806,625 to Woolf et al describes a telephone monitoring system having a plurality of monitors each connected in parallel to a separate telephone line to be monitored and communicating through parallel buss lines to a central record unit. Each monitor unit detects and decodes the telephone number dialed and performs all time-keeping functions. The central record unit sequentially polls each line and upon completion of the telephone activity sequentially retrieves from the line monitored the stored information for further processing, for example, storage or printing of the information regarding the telephone call. A principle drawback of the Woolf et al system is that the central record unit merely performs a polling function so that all logic and hardware for intelligence gathering must be located in each individual monitor unit. Each monitor unit, therefore, requires extensive intelligence gathering electronic circuitry. Furthermore, it would be extremely difficult to alter the Woolf et al monitor units so that they would accept dialed information without changing each individual telephone line circuit and the hard-wired data gathering device.